TPF-I SWG Report - Exoplanet Exploration Program - NASA

C HAPTER 4
4.9.5 Trade Study
A complete description of the trade study can be found in Lay et al. (2005). The scoring process is based
on the Kepner-Tregoe methodology. The idea is simple. The options are judged against a set of
mandatory criteria (“musts”) and a set of discriminators (“wants”). Each must is scored as a pass/fail for
each option, whereas the wants are weighted by importance and scored for each option on a scale of 1 to
10. Failing a mandatory criterion disqualifies the option from consideration. The highest weighted sum of
the discriminator scores determines the best option.
The options in this case are the six architectures described in the Section 4.3. The list of mandatory
criteria and discriminators was established through a lengthy process of iteration, based on initial inputs
from a joint TPF/Darwin workshop and then edited in a series of meetings of the TPF-I Architecture and
Table 4-3. Scores for Nulling Architectures Considered in Trade Study
Linear DCB X-Array (2:1) Diamond DCB Z-Array Triangle Linear 3
Overall score / 1000 816 860 789 749 779 795
Performance 381 385 337 341 292 342
Cost / risk 435 475 452 408 487 452
Table 4-4. Key Discriminators in Trade Study, with Scoring Relative to Linear DCB Array
Metric
Linear DCB
X-Array
(2:1)
Diamond
DCB
Z-Array Triangle Linear 3
Performance
Number of planet systems characterized by spectroscopy at
high angular resolution, beyond minimum required
Star count 0 0 -13 -4 -25 -19
Fidelity of image reconstruction
Redundancy / graceful degradation
rms/peak (in Point Spread Function for
planet at high res inner working angle)
Expected % of stars observed after loss
of 1 spacecraft
0 -2 -6 -8 -14 -4
0 4 -17 -6 -17 -17
General astrophysics potential Dynamic range of baselines (max/min) 0 -4 -5 -10 -8 -4
Cost and Risk
Number of types of spacecraft # types 0 14 0 -9 0 0
Mass margin Mass margin 0 0 3 3 15 6
Beam transport optics complexity # hops to combiner 0 12 0 -10 0 0
Beamcombiner optics complexity # parts (from schematic) 0 0 0 0 -10 -10
Number of mechanisms/ moving parts # mechanisms / deployments 0 0 4 4 11 4
Difficulty of integration and test # spacecraft 0 0 7 7 17 7
Complexity of flight operations # spacecraft 0 0 4 4 10 4
0 43 -27 -66 -33 -22
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